What I noticed while replacing the ac unit on my Jayco is the cold air enters a camber inside the roof at the ac unit. One duct is cut through the roof towards the curb side, that goes to the front, and one is cut towards the street side, and that one goes to the back. All registers are in a straight line, the ducts start from the sides of the ac unit and go front or back.
The Dutchmen we now have does the same thing. But unlike the Jayco that had a register in the "hall" between the bunks and bath door, the Dutchmen has a register in the rear bath and nothing in the rear bunks. The bathroom is like a meat locker and the bunks are warm. It would be nice to add a duct from the bath register and share with the bunk but it looks like a big job involving some major disassembling.
I think it would be a fairly simple job to cut through the ceiling, into a duct, and add a register. If the duct is there.